Wireless communication networks are steadily growing with an increasing number of systems for mobile communication being deployed. Today, wireless mobile communication networks based on various underlying Radio Access Technologies (RATs) are available. Second generation (2G) mobile communication systems, such as e.g. GSM/EDGE (Global System for Mobile Communications/Enhanced Data Rates for GSM Evolution), are based on a combined TDMA/FDMA (Time Division Multiple Access/Frequency Division Multiple Access) scheme. Third generation (3G) mobile communication systems, such as UMTS (Universal Mobile Telecommunications System) or CDMA2000 (IS-2000), are based on code division multiple access (CDMA) technology. Newer mobile communication technologies, such as e.g. WiMAX (Worldwide Interoperability for Microwave Access) or 3GPP (3rd Generation Partnership Project) Long Term Evolution (LTE), are based on orthogonal frequency division multiplexing (OFDM) technologies. LTE, e. g., uses OFDMA in the downlink and single carrier frequency division multiple access (SC-FDMA) in the uplink.
In various cellular communication systems, after power on, a mobile terminal a mobile terminal stays in a so-called Idle Mode until a request to establish an RRC (Radio Resource Control) connection is transmitted to the wireless network. In Idle Mode the Radio Network Controller (RNC) has no information on an individual mobile terminal, and can only address, for example, all mobile terminals in a radio cell or all mobile terminals monitoring a paging occasion. A mobile terminal may transit from Idle Mode to Connected Mode when an RRC connection is established, wherein the RRC connection may be defined as a point-to-point bidirectional connection between RRC peer entities in the mobile terminal and the wireless network, as, for example, the UMTS Terrestrial Radio Access Network (UTRAN) or the evolved UMTS Terrestrial Radio Access Network (eUTRAN) which is the air interface of LTE. Although embodiments of the present invention are in principle also applicable to other wireless communication systems, the following description will, only for exemplary reasons, focus on the wireless communication systems UMTS and LTE.
In the UMTS system, a Wideband Code Division Multiple Access (WCDMA) system, a mobile terminal, also referred to as User Equipment (UE) in the 3GPP standards, that has a RRC connection with a base station (NodeB), can transmit and receive user traffic in various RRC states, e.g. in the Cell_DCH (Dedicated Channel) and Cell_FACH (Forward Access Channel) states.
While the Cell_DCH state is characterized by an allocation of a circuit-switched Dedicated Physical Channel (DPCH) to the mobile terminal in uplink and downlink direction, respectively, wherein uplink refers to the communication direction from the mobile station to the base station, and downlink refers to the communication direction from the base station to the mobile station, no dedicated physical channel is allocated to the mobile station in Cell_FACH state, in which the mobile station continuously monitors a packet-switched Forward Access Channel (FACH) in the downlink. Packet switching is a digital networking communications method that groups all transmitted data—regardless of content, type, or structure—into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams (sequences of packets) over a shared network. When traversing network adapters, switches, routers and other network nodes, packets are buffered and queued, resulting in variable delay and throughput depending on the traffic load in the network. In Cell_FACH state the mobile station is also assigned a default common or shared transport channel in the uplink, as e.g. a common E-DCH (Enhanced Dedicated Channel) or a RACH (Random Access Channel) which it can use anytime according to the access procedure for that transport channel.
The RACH typically is a contention-based transport channel for initial uplink transmission, i.e., from the mobile terminal to the base station. The RACH function may be different depending on the technology of the system. This random access channel may be used for several purposes, as e.g. to access the network, to request resources, to carry control information, to adjust a timing-offset for the uplink, to adjust an uplink transmit power, etc. It may even be used to transmit small amounts of packet data. Contention resolution is the key feature of the RACH. Many mobile stations may attempt to access the same base station simultaneously, leading to collisions.
In the UMTS system, for example, a mobile terminal can make a random access to the uplink resources of a base station. This allows a base station to initiate a RRC connection to the network from an Idle Mode and send uplink transmissions in Cell_FACH state. The random access procedure comprises the mobile terminal decoding Broadcast Channel (BCH) information carried by the corresponding P-CCPCH (Primary Common Control Physical Channel) and finding out the RACH slots available as well as the scrambling code(s). The mobile terminal may then randomly select one of the available RACH resources, i.e. RACH slots, to be used for sending its preamble to the network. For this purpose the mobile terminal may set an initial power level to be used and transmit its preamble to the network. The mobile terminal may select a preamble signature randomly from a set of signatures. The RACH preamble in the UMTS system is of length 4096 chips and comprises of 256 repetitions of a signature of length 16 chips. There are a maximum of 16 available preamble signatures. All 16 preamble signature codes are available in every radio cell. After having sent the preamble, the mobile terminal waits for an Acquisition Indicator Channel (AICH) message from the base station, indicating to the mobile terminal whether it is allowed to proceed to a message part of the RACH procedure. If the AICH is negative, the mobile terminal will back off a random time, otherwise it will proceed to the message part. The mobile terminal transmits its uplink data traffic in the message part, which can be carried by the RACH or the common E-DCH. A 10 ms RACH message part radio frame is split into 15 slots, each of a length of 2560 chips. Each slot comprises two parts, a data part to which the RACH transport channel is mapped and a control part that carries Layer-1 control information. The data and control parts are transmitted in parallel. A 10 ms message part comprises one message part radio frame, while a 20 ms message part comprises two consecutive 10 ms message part radio frames.
If the mobile terminal fails to receive the AICH it will retry sending the preamble at a higher power level since the likely cause of the failure is due to the base station not having received the preamble. The mobile terminal will keep trying up to a maximum number of attempts to get an AICH from the base station. If it fails after the maximum number of attempts it will back off a random time before retrying. This RACH procedure is summarized by the flow chart of FIG. 7.
The mobile terminal may transmit the RACH preamble in access slots which are allocated or assigned to it. Such access slots will also be referred to as assigned access slots in the sequel. There are 15 access slots within two radio frames (2×10 ms), which are in principle available for random access. Such access slots will also be referred to as available access slots in the sequel. The 15 available access slots are divided into two 10 ms access slot sets or access slot groups, as shown in FIG. 8a. Here, the available access slots of a first group or Access Slot Set 1 (AS1) are numbered from 0 to 7, and the available access slots of a second group or Access Slot Set 2 (AS2) are numbered from 8 to 14.
The available access slots that a mobile terminal may actually use are assigned to the mobile terminal by the network. For example, in FIG. 8b, a mobile terminal is allowed to transmit its preamble in the assigned access slots {1, 4, 7, 10, 13}, wherein the assigned access slots {1, 4, 7} belong to access slot set 1 and the assigned access slots {10, 13} belong to access slot set 2. When a mobile terminal wishes to transmit a RACH preamble at a given time instant, it will conventionally wait for the next access slot set following in time and then randomly select a valid (i.e. assigned) access slot in said access slot set. Suppose the mobile terminal wishes to transmit a preamble (e.g. it suddenly got a packet to transmit) at the time corresponding to the available access slot 2 in AS1. The mobile terminal will hence randomly select an assigned access slot from the next access slot set, which is AS2. That is to say, it can (randomly) select either assigned access slot 10 or 13. Although access slots 4 and 7 of AS1 are also assigned to the mobile terminal, the mobile terminal cannot use them since they belong to the current access slot set (AS1), which has already begun. This causes unnecessary delay to the mobile terminal's initial access. Hence, it is desirable to reduce this delay.
A solution has been proposed wherein the mobile terminal is allowed to use remaining assigned access slots in the currently valid frame or access slot set rather than to wait for the next frame or access slot set. In the example of FIG. 8b, the mobile terminal may, hence, randomly select a remaining assigned access slot of AS1. That is to say, it may select either access slot 4 or acess slot 7, instead of having to wait for the next access slot set (AS2).
The problem with this solution is, however, that the mobile terminal will always have a smaller number of assigned access slots from which to choose in a current access slot set, since it is likely that the mobile terminal is in the middle of an access slot set when it wishes to transmit its RACH preamble. The rationale in selecting a random access slot within a whole access slot set is that the mobile terminals in a cell will be less likely to transmit their RACH preambles at the same time causing clashes leading to a failed attempt. The aforementioned solution will cause the mobile terminal to bias towards selecting assigned access slots that are towards the end of the current access slot set. In the example of FIG. 8b, mobile terminals will be more likely to select access slot 4 or 7 than access slot 1. When all mobile terminals perform the same procedure, the probablity of preamble clashes (especially towards the end of the access slot set) and, hence, failed attempts increases.
Hence, it is desirable to reduce random access delays and, at the same time, to also reduce the probablity of preamble clashes or collisions.